Emissions systems for compression-ignition (e.g., diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx).
With regard to reducing NO emissions, NO reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems utilize a reductant delivery system to introduce a reductant into the exhaust stream upstream of the SCR catalyst. When the proper amount of reductant is available at the SCR catalyst under the conditions, the reductant is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if a deficient amount of reductant is introduced into the exhaust stream upstream of the SCR catalyst, the SCR system may be unable to convert enough NOx.
The reductant delivery system may under-deliver the needed amount of reductant due to blockage of the reductant flow within the system. For example, there may be unintentional or intentional restrictions in the pressure line of the system. Alternatively, reductant may form deposits in the reductant delivery system (e.g., within the injector nozzle) that may restrict the flow of reductant through the system. In some implementations, a SCR system may provide data (e.g., one or more values) to an on-board diagnostic (OBD) system or unit to provide an alert indicating the failure of, or reduced operability of, a SCR system to convert enough NOx to meet a predetermined amount. One known indication of the inability of an SCR system to covert enough NO is the inability to deliver the necessary amount of reductant for NOx conversion due to blockage within the reductant delivery system. Known systems and associated diagnostics fail to adequately diagnose poor performance of reductant delivery systems due to blockage or other failures, and thus may fail to provide the data to the OBD system or unit regarding the malfunction of reductant delivery systems.